You're right, after rebooting the machine I tried again and got the same problems. Then this seems to be the same kind of problem you usually have in games like Oblivion, Skyrim or, in my case, Dark Souls 3. The FPS always takes a dive when the game needs to load data as you enter a new "cell" or zone. Once it's loaded for the first time, it's probably stored as "temp" data and would be wiped when you do a reboot. This would explain why I was never bothered by this; I'm way too used to it.
Then there's not much we can do since the only real solution would be to play the game on a beefier rig - better CPU, more VRAM, and so on.
Last edited by Try2Handing; 31/12/17 12:51 AM.